1. Technical Field
The present disclosure generally relates to the field of power amplifiers. More particularly, the present disclosure relates to an electronic circuit for driving a switching power amplifier for a diffuser of an audio signal.
2. Description of the Related Art
It is known that an audio amplification system comprises an audio signal source (for example, a Compact Disk wherein the audio signal is of the Pulse Code Modulation—PCM-type), an audio amplifier, a low-pass filter for eliminating frequencies higher than the audio band and an audio signal diffuser (for example, a speaker, headphones or earphones.)
More particularly, the audio amplifier comprises a Pulse Width Modulator—PWM-, comprises a driving circuit of a switching amplifier and comprises the switching amplifier.
It is known that proper precautions should be taken in order to avoid (or reduce) abrupt transitions at the ends of the diffuser of the audio signal when switching on the switching amplifier (i.e., when the switching amplifier is powered-up) and when switching off (i.e., when the switching amplifier is powered-down), since abrupt transitions cause the generation of audible noise when switching on and off the switching amplifier; said noise is commonly referred to as “pop” noise (or also “click” noise).
A prior art for reducing the generation of the pop noise is disclosed in the US patent application No. 2008/0030270, wherein (see for example FIG. 4a and par. 22) the voltage (in case the switching amplifier is switched on) at the ends of the speaker is step-wise increased in a switch-on transient phase comprised between the instant when the switching amplifier is switched on and a steady instant wherein the switching amplifier is normally operating. The Applicant has observed that a drawback of this prior art is that it is too complex since it requires an accurate control of the time instants (see formulae in par. 24); moreover this prior art is not always efficient in reducing the pop noise.
Another prior art for reducing the pop noise generation is disclosed in the US Patent Application No. 2006/0262843, wherein in the transient phase (for example, when switching on) the pulse width modulator generates a pair of pulse signals with width gradually increasing linearly or exponentially (see for example FIGS. 5A and 5B). The Applicant has observed that a drawback of this prior art is that it is not efficient in the reduction of the pop noise.
A further prior art for reducing the pop noise generation is disclosed in the US patent application No. 2005/0083115, wherein (see for example FIG. 6D) the voltage differential signal in output from the full bridge switching amplifier is a square wave with a pulses width gradually increasing starting from value zero and with amplitude of subsequent pulses alternating between the positive value and the negative value. The Applicant has noticed that a drawback of this prior art is that it is too complex.
A further prior art for reducing the pop noise generation is disclosed in the U.S. Pat. No. 6,489,840, wherein (at the switch-on) a square wave periodic signal is generated in output from the amplifier with a pulse whose width is a fraction of the period, preferably ¼ of the period. The Applicant has noticed that a drawback of this prior art it that it is capable of reducing the pop noise only by a predetermined amount.